Phase-Sequenced Deposition of Calcium Titanate/Hydroxyapatite Films with Controllable Crystallographic Texture onto Ti6Al4V by Triethyl Phosphate-Regulated Hydrothermal Crystallization

The aim of this study was to investigate the use of triethyl phosphate (TEP) to regulate the hydrothermal crystallization of hydroxyapatite (HA) films onto Ti6Al4V substrates. The growth mechanism of the HA film and the development of [0001] HA crystallographic texture were studied. Films were crystallized in a 0.232 m Ca(NO 3 ) 2 −0.232 m EDTA−0.187 m TEP−1.852 m KOH−H 2 O chemical system with a final isothermal temperature of 200 °C, and then evaluated at synthesis times from 0 to 46 h by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and X-ray pole figures. Thermodynamic phase stability diagrams were calculated to validate experimental findings. XRD, FESEM, TEM, and EDX results demonstrated the crystallization of a CaTiO 3 film below 180 °C and a HA film above 180 °C. FESEM and X-ray pole figure analysis revealed a refinement of the orientation of the (0002) HA crystallographic plane and the c -axis of hexagonal single crystals of HA, [0001], with increasing synthesis time. On the basis of these results, it is concluded that the use of TEP-regulated hydrothermal crystallization enables the deposition of CaTiO 3 and then HA in a single, phase sequenced process, the first such process reported in the hydrothermal HA literature. The HA film is deposited by means of a competitive growth mechanism that enables the [0001] crystallographic orientation of hexagonal single crystals to be engineered with synthesis time.